A light-emitting diode (LED) has merits in terms of the light efficiency and the durability and, therefore, draws attention as a light source for a backlight of an illumination device or a display device.
The light-emitting diode is driven at a low direct current. Thus, in the related art, there has been used a power supply device for converting a commercial AC voltage (AC 220 V) to a DC voltage. For example, use has been made of a SMPS (Switched-Mode Power Supply), a linear power, etc. However, this power supply device is usually low in the conversion efficiency. Among the components used, an electrolytic capacitor is short in the lifespan. Thus, the use of the power supply device poses a problem of shortening the lifespan of a light-emitting diode illumination device.
In order to solve this problem, there has been developed a method in which two light-emitting diode strings are directly connected to an AC power supply in a forward direction and a reverse direction without performing DC conversion. However, this method suffers from a problem in that only 50% or less of the connected light-emitting diodes are turned on, consequently exhibiting low efficiency. Furthermore, a current flowing through the light-emitting diode is abruptly changed as a result of a change in the magnitude of an input voltage. This may adversely affect the light-emitting diode elements and may pose a problem in that a change in the brightness is large. In addition, a current is allowed to flow through a circuit only when the magnitude of an input voltage is equal to or larger than a value capable of operating all the light-emitting diodes included in the light-emitting diode strings. For that reason, a waveform difference between an alternating current flowing through the circuit and an AC voltage is large. This poses a problem in that the power factor is reduced.
In order to solve the problem inherent in the method of directly using an AC power supply, there have been developed different methods in which an alternating current is used after rectification through a bridge circuit. For example, Japanese Patent No. 4581646, U.S. Pat. No. 6,989,807, Japanese Patent Application Publication No. 2011-040701 and Korean Patent Application Publication No. 10-2012-0041093 discloses a method in which, after rectifying an AC voltage, the number of light-emitting diodes applied with the rectified voltage is adjusted depending on the change in the magnitude of the rectified voltage. In this method, as compared with a method of directly using an AC power supply, the number of operating light-emitting diodes increases. Therefore, this method has an advantage in that the efficiency is high and the current supply time is short, thereby improving the power factor.
The method of using an AC power supply after rectifying the same by means of a bridge circuit has a problem in that, since the light-emitting diodes are driven by a full-wave rectified wave having a frequency of 120 Hz, the magnitude of an AC power supply becomes equal to or smaller than a light-emitting diode driving voltage in a significant interval around a phase of 180 degrees, consequently generating a lighting failure.
Human eyes recognize a light source flickering at a flicker fusion frequency or higher as a continuously turned-on light source rather than an intermittently flickering light source. Accordingly, a light-emitting diode flickering at a flicker fusion frequency or higher is felt by human eyes as if it is continuously turned on. Most of the human eyes recognize a light source flickering at 75 Hz or higher as a continuously turned-on light source. However, a light-sensitive person may recognize the flicker of a light-emitting diode flickering at 120 Hz and, therefore, may suffer from photo-seizure. For that reason, it is preferred that the light-emitting diode flickers at a high frequency as far as possible.
In Japan, it is stipulated in the lighting certification standard that no flicker phenomenon should be generated between 100 Hz and 500 Hz. European countries are trying to stipulate that a lighting fixture should be driven at a frequency of 150 Hz or higher. In recent years, U.S.A. provides the energy star certification provision which prescribes that a lighting fixture having a flicker level not exceeding a predetermined level should be excluded from a certification candidate. Under these circumstances, there may be a situation that it is impossible to sell a light-emitting diode driven by a full-wave rectified wave.
In order to ameliorate this situation, Korean Patent Application Publication No. 10-2010-0104362 discloses a method which makes use of a valley fill circuit. This method is capable of providing a flicker phenomenon improvement effect. However, a capacitor having a large capacity has to be used in this method. Use of the capacitor poses an adverse effect in that the power factor becomes poor.
As another improvement method, it may be possible to use a charging/discharging circuit disclosed in Korean Patent Application Publication No. 10-2012-0082468. In this method, a flicker phenomenon is ameliorated. However, this method fails to overcome a limit that a flicker is generated at a frequency of 120 Hz. Moreover, if an input voltage decreases, charging is not sufficiently carried out and a discharging start point becomes shorter. Thus, the flicker phenomenon is conspicuous.
The method of using an AC power supply after rectifying the same by means of a bridge circuit has another problem. Specifically, if a driving voltage is set high, a phase interval where a light-emitting diode is turned on becomes small. This reduces the light-emitting diode use efficiency (the effective power consumption of the light-emitting diode/the power consumption of the light-emitting diode during the DC rated current operation) and the power factor. If a driving voltage is set low, a significant amount of electric power is consumed as heat and the power supply efficiency is reduced.
Korean Patent Application Publication No. 10-2012-0074502 discloses a lighting device provided with a charging/discharging block. In a charging interval, the charging/discharging block charges electric charges at a drive terminal. The charging/discharging block is discharged at a voltage equal to or less than a driving voltage of a light-emitting diode array, thereby removing an interval where the light-emitting diode array is turned off.
As a further method for improving the flicker phenomenon, there is available a method of increasing the flicker frequency of a light-emitting diode. U.S. Pat. No. 8,299,724 discloses a method in which a current flowing through a light-emitting diode array is cut off by an OVP (over-voltage protection) element when a drive terminal voltage is at a peak value, thereby increasing the flicker frequency of the light-emitting diode array to become four times as high as an input AC power supply frequency. However, this method suffers from a problem in that, if the drive terminal voltage is equal to or lower than the driving voltage of the light-emitting diode array, a turn-off interval becomes longer.
Furthermore, U.S. Patent Application Publication No. 2012-0229041 discloses a method in which electric energy is stored by means of an energy storing element such as a capacitor or the like. If the magnitude of a drive terminal voltage becomes equal to or smaller than a driving voltage of a light-emitting diode array, the energy storing element is discharged so that the frequency of a current applied to the light-emitting diode array becomes four times as high as an input AC power supply frequency.
In the meantime, as a method of realizing a high-efficiency lighting device and consequently saving electric energy, an attempt has been made to consider the aspect of psychophysics which studies the relationship between a cognitive phenomenon and a physical property of a stimulus.
In general, the amount of light energy generated in a lighting device is increased in proportion to the amount of input electric energy. However, it is another matter how human eyes recognize the light.
A light-emitting diode (LED) is controlled by a constant current control method which makes use of a DC power supply or a pulse width modulation (PWM) control method which makes use of a pulse voltage. The pulse width modulation control method is a control method in which electric power is controlled by adjusting a pulse frequency and a duty cycle.
Results of studies on how human eyes recognize the brightness of an intermittently flickering light source have been announced from 1900s.
According to the Talbot-Plateau law, it is said that a human who observes an intermittently flickering light source recognizes the light source as if it is continuously turned on at an average brightness.
Furthermore, according to the Broca-Sulzer law, it is said that when exposed to strong light such as camera flash light or the like, human eyes feel the light several times as bright as the actual light brightness.
According to the recent study conducted at Ehime University in Japan, it is said that if a pulse voltage is used, the Broca-Sulzer effect has a larger influence than the Talbot-Plateau effect, whereby human eyes recognize a light source to be brighter than an average brightness.
Moreover, according to the study conducted at Tianjin University in the People's Republic of China, it is said that if the average intensity remains the same as illustrated in FIG. 13, an LED driven by a PWM control method is felt brighter than an LED driven by a constant current control method.
In FIG. 13, the circle-plotted curve indicates the apparent brightness of a light emitting diode driven by a constant current control method and the triangle-plotted curve indicates a change in the apparent brightness depending on a change in a duty cycle of a light emitting diode driven by a PWM control method in the case where the PWM control method equal in average intensity to the constant current control method. The term “apparent brightness” refers to a psychological quantity of the contrast corresponding to the brightness which is a physical quantity of light. That is to say, the apparent brightness means a human-felt brightness rather than a real brightness.
Furthermore, as shown in FIG. 13, it can be noted that if a pulse voltage having a shorter duty cycle is used, a difference in apparent brightness between the PWM control method and the constant current control method becomes larger.
Referring to FIG. 13, it can be appreciated that if the frequency is 100 Hz and if the duty cycle is 50%, the light is felt about 40% brighter in the PWM control method than in the constant current control method. It can also be seen that if the duty cycle is 80%, the light is felt about 25% brighter in the PWM control method than in the constant current control method. It can be further noted that if the duty cycle is 100%, no difference in the brightness exists between the PWM control method and the constant current control method.
These results can also be confirmed in the study conducted at Ehime University in Japan. According to the study conducted at Ehime University, it is said that if an LED is driven at a duty cycle of 5% and at a pulse voltage of 60 Hz, the light is felt 120% brighter at the most in a PWM drive method than in a constant current drive method.
It can be expected from the results illustrated in FIG. 13 that if the average intensity remains the same, an LED driven by a pulse voltage having a larger intensity and a shorter duty cycle will be felt brighter than an LED driven by a pulse voltage having a smaller intensity and a longer duty cycle.